PROBLEMS OF THE ARTESIAN WATER SUPPLY OF AUSTRALIA. 113 



witliin which the higher temperatures are met with cor- 

 responds with a deep valley in the submerged " bed-rock 

 surface," while the area which is characterised by cooler 

 conditions is coincident with an underground ridge of 

 quartzite, which, in fact, outcrops at the surface in places. 

 There are indications at the north side of the buried ridge 

 that the quartzite is underlain by granite. 1 



It seems possible that these variations in the temperature 

 gradient may be clue to the irregularities in the submerged 

 bed-rock surface, and to the differences between the con- 

 ductivities and thermal capacities of the bed-rock and of the 

 overlying water-bearing series respectively, though it is 

 noteworthy that the author of the paper quoted considers 

 that all the explanations which have yet been suggested, 

 including that of "different ratios of conductivity" are 

 unsatisfactory. 



The abnormally high temperatures referred to by Gregory 

 as occurring in some of the Australian bores may also be 

 possibly due to the bores having been put down in proximity, 

 in some cases, to centres of expiring volcanic activity. The 

 Triassic rocks along the coast of New South Wales were 

 intruded, during Tertiary times, by many dolerite dykes, 

 and there is evidence all over the eastern States that vol- 

 canic activity was very wide-spread during that period. 

 It is therefore only reasonable to suppose that the porous 

 Triassic sandstones which form the base of the artesian 

 basin have also been intersected by many dolerite dykes 

 which do not appear at the present surface because they 

 are older than the Post-Tertiary deposits of which it is 

 formed. As a matter of fact specimens of dolerite have 

 been brought up with the drillings from some of the bores. 



It is admitted that it is difficult to find a perfectly satis- 

 factory explanation of these differences in temperature, 



1 N. H. Darton, 17th Ann. Kept. U.S. Geol. Sur., 1895-6, Pt. n., p. 673. 

 H— Aug. 7, 1907. 



